This invention relates to a purification process for removing impurities from a solution that is subsequently used for example in the fabrication of photoreceptors.
Various polymers are used in bulk in forming the layers of a photoreceptor. Examples of polycarbonates, polycarbazoles, polyarylates, and copolyesters that are suitable for use in photoreceptors are discussed in Mishra et al., U.S. Pat. No. 5,607,802 and Bergfjord, Sr. et al., U.S. Pat. No. 5,686,215, the disclosures of which are totally incorporated herein by reference.
These polymers are sometimes contaminated with trace impurities that adversely affect the electrical characteristics of the resulting photoreceptors. The contaminated materials must then be discarded, returned to the supplier, or used for non-critical layers such as the anticurl layer. Sometimes, the trace impurities cannot be identified, or if amenable to identification, the identification process may be costly. Thus, there is a need, which the present invention addresses, for a relatively inexpensive process to purify those polymers that are used in fabricating a photoreceptor. Of course, the polymers described herein can be used for other purposes and in embodiments of the present invention, these polymers as well as the instant purification process are not limited to the field of photoreceptors.
The present purification process involves the use of an adsorbent such as alumina or magnesium sulfate. A number of known uses of alumina and magnesium sulfate are described in xe2x80x9cReagents for Organic Synthesis,xe2x80x9d Louis F. Fieser and Mary Fieser, John Wiley and Sons, New York (1967), pp.19, 20 and 634.
A conventional purification technique is disclosed in Sando et al., U.S. Pat. No. 5,180,497.
The present invention is accomplished in embodiments by providing a purification process comprising: contacting a contaminated composition comprised of an optional fluid, a polymer, and an impurity with an adsorbent composition including an adsorbent, wherein the adsorbent is selected from the group consisting of an alumina and a magnesium sulfate, wherein a portion of the impurity binds to the adsorbent, wherein the polymer is a polycarbonate, a carbazole, a polyarylate, or a copolyester having the formula 
where n is the degree of polymerization.
The present purification process involves contacting the contaminated composition with the adsorbent composition. In one embodiment, the contaminated composition is passed through a column packed with the adsorbent composition where at least a portion of the impurity binds to the adsorbent in the column. After passing through the column, the purified solution is substantially or totally free of any impurity. Anhydrous magnesium sulfate was obtained as a fine powder from Fisher Scientific Company with the catalog number M65-500. Neutral alumina (activity 1) was obtained 29 from Aldrich Chemical Company (catalog number 19,997-4) with a reported mesh size of 150 (58 Angstroms), a surface area of 155 meters2 per gram, and with a pH 7.0 (+/xe2x88x920.5) as an aqueous dispersion. Similarly, activated acidic alumina (Aldrich, catalog number 19,996-6) with a pH 4.5 (+/xe2x88x920.5) as an aqueous dispersion, and activated basic alumina (Aldrich catalog number 19,944-3) with a pH 9.5 (+/xe2x88x920.5) as an aqueous dispersion, were obtained with comparable mesh size and surface areas. Alternatively, suitable Woelm acidic and basic aluminas were obtained from ICN Pharmaceuticals, Inc., Life Sciences Group, 26201 Miles Road, Cleveland, Ohio 44128, or Waters Associates, 61 Fountain St., Farmington, Mass. 01701. The use of a column is preferred with the alumina.
Using any of the purification processes described herein, the contaminated composition contacts the adsorbent composition for a time effective for binding a significant portion of the impurity to the adsorbent, preferably at least about 30 minutes, more preferably at least about 4 hours, and especially for a time ranging from about 4 hours to about 16 hours. An optional vacuum may be applied to the contaminated composition during the purification process.
In another embodiment, one forms a slurry of the contaminated composition and the adsorbent composition, thereby resulting in a liquid portion and a solids portion in the slurry, and then separating the liquid portion from the solids portion by for example centrifuging or filtering. At least a portion of the impurity binds to the adsorbent where the impurity and the adsorbent are found in the solids portion. After separation of the liquid portion from the solids portion of the slurry, the liquid portion is substantially or totally free of any impurity.
In embodiments of the present invention, either the contaminated composition, the adsorbent composition, or both may contain a fluid. The fluid in the contaminated composition may be the same or different from the fluid in the adsorbent composition. The fluid in the contaminated composition is preferably a solvent for the polymer such as one or more of the organic liquids described herein. The fluids for the contaminated composition and the adsorbent composition may be an organic liquid such as chlorinated liquids like 1,1,2,2-tetrachloroethane, methylene chloride, and monochlorobenzene; toluene; tetrahydrofuran; polar aprotic liquids such as dimethyl formamide, dimethylacetamide, N-methylpyrrolidinone, and dimethylsulfoxide; water; or mixtures thereof. The preferred fluid for the adsorbent composition is water; magnesium sulfate is soluble in the water, whereas alumina is generally insoluble in water and in many other liquids. The fluid in the adsorbent composition may be a solvent for the polymer.
The contaminated composition may be composed entirely of the polymer and the impurity. If a fluid is present, the amount of polymer in the contaminated composition ranges for example from about 1 to about 50 weight percent, preferably from about 5 to about 20 weight percent, based on the weight of the contaminated composition.
The adsorbent composition may be composed entirely of the adsorbent. If a fluid is present, the amount of adsorbent in the adsorbent composition ranges for example from about 10 to about 95 weight percent, preferably from about 20 to about 70 weight percent, based on the weight of the adsorbent composition.
The anhydrous magnesium sulfate used was a fine powder and was used as received from Fisher Scientific. Because impurity levels are so low (parts per million or less), it is not known how much adsorbent is actually required. Typically, between about 50 and 100 weight percent magnesium sulfate, based on the weight of resin in solution, was used. However, the most important factor in the removal of the impurity is the time of contact between the adsorbent and the polymer in solution. For example, when polycarbonate (50 grams), contaminated with about 10 parts per million of polyethylene oxide (PEO, Scientific Polymer Products catalog number 491, with 7,000 molecular weight), at 10 weight percent solids in methylene chloride, was stirred with 50 grams of magnesium sulfate for about 10 minutes and then the dispersion was rapidly filtered through a sintered glass funnel, the PEO contaminant was not removed, as determined by xerographic testing of photoreceptor devices made with the material. When the same experiment was repeated except that the magnesium sulfate treatment time with the polymer solution was extended to 4 hours, the PEO impurity was completely removed as determined by xerographic testing. Thus, about 10 minutes contact between the magnesium sulfate and the polymer solution may be inadequate, whereas about four hours contact, and preferably about 16 hours contact, was satisfactory to remove all the PEO impurity (to less than 1 part per billion). The slurry method is preferred for the magnesium sulfate for adsorptive filtration and removal of impurities. The magnesium sulfate contained not only polyethylene oxide but also trace amounts of other contaminants including for example one or more of the following: polydimethylsiloxane, amylene, chlorocarbons, and unidentified black and brown colored ferromagnetic impurities.
For comparison, a water wash of the polymer solution, filtration through fluted filter paper, treatment with potassium carbonate (50 grams per 50 grams polycarbonate as a 5 wt. % solids in methylene chloride), and reprecipitation of the polycarbonate in anhydrous methanol were all unsatisfactory methods to remove the polyethylene oxide (PEO) impurity from the contaminated polycarbonate. When the PEO contaminated polycarbonate (5 grams) in methylene chloride (90 grams) was passed drop-wise through a 100-milliliter burette (with a 0.75-inch diameter) packed with 60 milliliters of acidic alumina (activity grade 1, Woelm acid grade (Waters Associates, 61 Fountain St., Farmington, Mass. 01701) by eluting generously with methylene chloride, no trace of the PEO contaminant remained. These experiments were repeated with neutral alumina (Aldrich catalog number 19,997-4) and basic alumina (Woelm), respectively, with comparable, successful results.
Fluid rate flow (through-put) in the column was also important. A column (Aldrich pressure filter funnel catalog number Z14,767-2) was packed with about 4-inches of neutral alumina (Aldrich). Thirty minutes of treatment time was required to remove all the PEO contaminant from 100 milliliters of a 5 weight percent solids solution of the PEO contaminated polycarbonate in methylene chloride by generous elution with methylene chloride. When the flow rate was increased by vacuum filtration at 5 psi (gauge), 6 minutes to treat 100 milliliters of the same polymer solution showed only a marginal improvement, and 1 minute to treat 100 milliliters of the polymer solution (at 15 psi, gauge) was totally unsatisfactory, as determined by xerographic measurements.
The magnesium sulfate and alumina purification methods described above were also applied to polyvinylcarbazole, Ardel polyarylate (Amoco), and polyester Mor-Ester 49,000 (Morton-Thiokol, Inc.). Metal ion contaminants were markedly reduced in all the purified samples of these polymers. The acid number of Ardel polyarylate was reduced from 1.48 milligrams of hydroxide per gram of resin to 0.65 mg OH31  per gram by basic alumina treatment, and to 0.31 mg OHxe2x88x92 per gram by neutral alumina treatment. The antimony oxide catalyst contaminant in Mor-Ester 49,000 was somewhat lowered (but not completely removed) by a column treatment with neutral alumina. The alumina treatment was therefore followed with a magnesium sulfate purification treatment to eliminate catalyst resides from the polyester.
In the present invention, alumina, magnesium sulfate, or both may be used as the adsorbent. If used together, the alumina and magnesium sulfate may be used in any suitable ratio such as 50:50 by weight. The adsorbent such as alumina may have an acidic, neutral or basic pH as an aqueous dispersion of the adsorbent in water. The magnesium sulfate may be anhydrous and slightly acidic.
Prior to use of the present process, the contaminated composition may have an impurity level in the parts per million (xe2x80x9cppmxe2x80x9d) level, such as from about 5 to about 200 ppm or even less, such as in the parts per billion range. Polyethylene oxide contaminant was found to affect trigonal selenium photoreceptors at levels as low as 1 part per billion in polycarbonate used for the charge transport layer of the device. The present invention reduces the level of impurity in the solution to a xerographically undetectable level. The impurity may be a salt, a polar material, a surfactant, or a mixture thereof. Examples of a salt impurity are a metal salt such as iron oxides, antimony oxide, ruthenium chloride, and the like. Examples of a polar material include glycol such as ethylene glycol, water, organic acids such as adipic acid and acetic acid, amines such as triethylamine, and the like. Examples of a surfactant include polydimethylsiloxane, polyethylene oxide, Triton surfactants, quaternary ammonium compounds, and the like.
In the case of photoreceptors with a trigonal selenium photogenerator (and also those with benzimidazole perylene photogenerator), contaminants such as polyethylene oxide in the polycarbonate charge transport layer can cause the photoreceptor discharge potential (i.e., the residual voltage after light exposure) to increase with use at contamination levels as low as 1 part per billion. This condition, termed photoreceptor cycle-up or loss in cleaning field, markedly affects copy quality. In charge area development (CAD), background areas are prevented from developing toner (dark background) by biasing the development roll at about 100 to 150 volts above the potential of the white background discharge areas. This voltage potential difference is termed the cleaning field. If the photoreceptor is unstable and cycles-up during use (due to impurities that affect the photogenerator pigment or cause charge trapping in other layers of the device), the discharge potential of the background area creeps up during use (cycle-up). With fixed bias on the development roll, the difference on the cleaning field (potential) goes down causing toner to stick to the background areas. This unsatisfactory condition shows up as a slightly dark region in an otherwise white print. Thus, photoreceptor materials must be reliably pure to ensure consistent print quality performance.
Suitable polycarbonate resins include, but are not limited to, resins having a molecular weight from about 20,000 to about 120,000, more preferably from about 50,000 to about 100,000. Examples of such polycarbonate resins are poly(4,4xe2x80x2-diopropylidene-diphenylene carbonate) with a molecular weight of about 35,000 to about 40,000, available as LEXAN 145(trademark) from General Electric Company; poly(4,4xe2x80x2-isopropylidene-diphenylene carbonate) with a molecular weight of about 40,000 to about 45,000 available as LEXAN 141(trademark) from General Electric Company; a polycarbonate resin having a molecular weight of from about 50,000 to about 100,000, available as MAKROLON(trademark) from Farben Fabricken Bayer A. G.; a polycarbonate resin having a molecular weight of from about 20,000 to about 50,000 available as MERLON(trademark) from Mobay Chemical Company; polyether carbonates; and 4,4xe2x80x2-cyclohexylidene diphenyl polycarbonate. Polycarbonate polymers suitable for practicing this invention also can be made, for example, from 2,2-bis(4-hydroxyphenol)propane; 4,4xe2x80x2-dihydroxy-diphenyl- 1,1-isobutane; 4,4xe2x80x2-dihydroxy-diphenyl-4,4-heptane; 4,4xe2x80x2-dihydroxy-diphenyl-2,2-hexane; 4,4xe2x80x2-dihydroxy-triphenyl-2,2,2-ethane; 4,4xe2x80x2-dihydroxy-diphenyl-1,1-cyclohexane; 4,4xe2x80x2-dihydroxy-diphenyl-beta-beta-decahydronaphthalene; 4,4xe2x80x2-dihydroxy-diphenyl-sulphone and the like. High heat resistant polyphthalate carbonate resins such as LEXAN 4701(trademark) and 4501(trademark) available from General Electric Company and the like are also suitable.
Typical carbazole polymers include, for example, polyvinylcarbazole and polyvinylcarbazole derivatives. Preferably, the carbazole polymers are selected from the group consisting of polymers having the structural formulae (A), (B), (C) and (D) below: 
wherein n, degree of polymerization, is a number between about 800 and about 6,000. The polymer may comprise a single carbazole polymer or a mixture of carbazole polymers.
A typical polyarylate has repeating units represented in the following formula: 
wherein R is C1-C6 alkylene, preferably C3 as in isopropylidene. These polyarylates are solvent soluble and have a weight average molecular weight greater than about 5,000 and preferably greater than about 30,000. The preferred polyarylate polymers have recurring units of the formula: 
The phthalate moiety may be from isophthalic acid, terephthalic acid or a mixture of the two at any suitable ratios ranging from about 99 percent isophthalic acid and about 1 percent terephthalic acid to about 1 percent isophthalic acid and about 99 percent terephthalic acid, with a preferred mixture being between about 75 percent isophthalic acid and about 25 percent terephthalic acid and especially between about 50 percent isophthalic acid and about 50 percent terephthalic acid. The polyarylates ARDEL(trademark) from Amoco and DUREL(trademark) from Celanese Chemical Company are preferred polymers. The most preferred polyarylate polymer is available from the Amoco Performance Products under the tradename ARDEL(trademark) D-100. ARDEL(trademark) is prepared from bisphenol-A and a mixture of 50 mole percent each of terephthalic and isophthalic acid chlorides by conventional methods. ARDEL(trademark) D-100 has a melt flow at 375xc2x0 C. of 4.5 g/10 minutes, a density of 1.21 Mg/m3, a refractive index of 1.61, a tensile strength at yield of 69 MPa, a thermal conductivity (k) of 0.18 W/mxc2x0K and a volume resistivity of 3xc3x971016 ohm-cm. DUREL(trademark) is an amorphous homopolymer with a weight average molecular weight of about 20,000 to about 200,000. Two or more different polyarylates may be used. These polyarylates are disclosed in U.S. Pat. No. 5,492,785, the entire disclosure thereof being incorporated herein by reference.
Any suitable copolyester film forming resin may be used. An especially preferred copolyester is a linear saturated copolyester reaction product of four diacids and ethylene glycol. The molecular structure of this linear saturated copolyester has the following structural formula: 
where n is the degree of polymerization which is between about 170 and about 370. The mole ratio of diacid to ethylene glycol in the copolyester is 1:1. The diacids are terephthalic acid, isophthalic acid, adipic acid and azelaic acid. The mole ratio of terephthalic acid to isophthalic acid to adipic acid to azelaic acid is 4:4:1:1. A representative linear saturated copolyester of this structure is commercially available as Mor-Ester 49,000 (available from Morton International Inc., previously available from duPont de Nemours and Co.). The Mor-Ester 49,000 is a linear saturated copolyester which consists of alternating monomer units of ethylene glycol and four randomly sequenced diacids in the above indicated ratio and n in the structural formula has a value which gives a weight average molecular weight of about 70,000. This linear saturated copolyester has a Tg of about 32xc2x0 C. Another preferred representative polyester resin is a copolyester resin having the structural formula below where the diacid is selected from the group consisting of terephthalic acid, isophthalic acid, and mixtures thereof; the diol is selected from the group consisting of ethylene glycol, 2,2-dimethyl propane diol and mixtures thereof; the ratio of diacid to diol is 1:1; n is a number between about 175 and about 350 and the Tg of the copolyester resin is between about 50xc2x0 C. about 80xc2x0 C. Typical polyester resins having the above structure are commercially available and include, for example, Vitel PE-100, Vitel PE-200, Vitel PE-200D, and Vitel PE-222, all available from Goodyear Tire and Rubber Co. More specifically, Vitel PE-100 polyester resin is a linear saturated copolyester of two diacids and ethylene glycol where the ratio of diacid to ethylene glycol in this copolyester is 1:1. The diacids are terephthalic acid and isophthalic acid. The ratio of terephthalic acid to isophthalic acid is 3:2. The Vitel PE-100 linear saturated copolyester consists of alternating monomer units of ethylene glycol and two randomly sequenced diacids in the above indicated ratio and has a weight average molecular weight of about 50,000 and a Tg of about 71xc2x0 C. This copolyester is represented by the following formula: 
wherein the diacid is selected from the group consisting of terephthalic acid, isophthalic acid, and mixtures thereof,
the diol comprises ethylene glycol and 2,2-dimethyl propane diol,
the mole ratio of diacid to diol is 1:1, the mole ratio of terephthalic acid to isophthalic acid is 1.2:1, the mole ratio of ethylene glycol to 2,2-dimethyl propane diol is 1.33:1,
n is a number between about 160 and about 330, and the Tg of said copolyester resin is between about 50xc2x0 C. and about 80xc2x0 C.
Another polyester resin, represented by the above formula, is Vitel PE-200 available from Goodyear Tire and Rubber Co. This polyester resin is a linear saturated copolyester of two diacids and two diols where the ratio of diacid to diol in the copolyester is 1:1. The diacids are terephthalic acid and isophthalic acid. The ratio of terephthalic acid to isophthalic acid is 1.2:1. The two diols are ethylene glycol and 2,2-dimethyl propane diol. The ratio of ethylene glycol to dimethyl propane diol is 1.33:1. The Goodyear PE-200 linear saturated copolyester consists of randomly alternating monomer units of the two diacids and the two diols in the above indicated ratio and has a weight average molecular weight of about 45,000 and a Tg of about 67xc2x0 C.
Any suitable solvent may be used. Typical solvents include tetrahydrofuran, toluene, hexane, cyclohexane, cyclohexanone, methylene chloride, 1,1,2-trichloroethane, monochlorobenzene, chloroform, N-methylpyrrolidinone, N,N-dimethylformamide, N,N-dimethylacetamide, and the like, and mixtures thereof. Any suitable ratio of the solvent to the polymer may be employed; the polymer may be present in the solvent at a concentration ranging for example from about 1% to about 20% by weight of the solvent.
The invention will now be described in detail with respect to specific preferred embodiments thereof, it being understood that these examples are intended to be illustrative only and the invention is not intended to be limited to the materials, conditions, or process parameters recited herein. All percentages and parts are by weight unless otherwise indicated.
A number of the Examples below relates to the preparation of bisimidazole perylene (xe2x80x9cBZPxe2x80x9d) photogenerator layers. Specifically, the benzimidazole perylene was dispersed with PCZ-polycarbonate binder in tetrahydrofuran. A photogenerating layer of BZP, which is preferably a mixture of bisbenzimidazo(2,1-a-1xe2x80x2,2xe2x80x2-b)anthra(2,1,9-def:6,5,10-dxe2x80x2exe2x80x2fxe2x80x2)diisoquinoline-6,11-dione and bisbenzimidazo(2,1-a:2xe2x80x2,1xe2x80x2-a)anthra(2,1,9-def:6,5,10-dxe2x80x2exe2x80x2fxe2x80x2)diisoquinoline-10, 21-dione, is taught in U.S. Pat. No. 4,587,189. There are also disclosed in U.S. Pat. No. 3,871,882 photoconductive substances comprised of specific perylene-3,4,9,10-tetracarboxylic acid derivative dyestuffs. In accordance with this patent, the photoconductive layer is preferably formed by vapor depositing the dyestuff in a vacuum. Also, there are specifically disclosed in this patent dual layer photoreceptors with perylene-3,4,9,10-tetracarboxylic acid diimide derivatives, which have spectral response in the wavelength region of from 400 to 600 nanometers. In U.S. Pat. No. 4,587,189, there is illustrated a layered imaging member with, for example, a BZP perylene, pigment photogenerating component. Both of the aforementioned patents disclose an aryl amine component, such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methyl phenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine dispersed in a polycarbonate binder, as a hole transport layer.